The 4640 has a Dolby Digital decoder, while the 4660 does not. If your video file has a version of Dolby Digital that your TV can't play, the 4660 can't convert it. For example, if your media has Dolby Digital+, your TV probably can't play it. Your TV can certainly play standard AC-3, so I would change your audio settings from Stereo-PCM to DD and try that.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.